This invention relates to a filter underdrain apparatus.
Filter underdrain apparatuses are by no means new. Examples of such apparatuses are found in U.S. Pat. Nos. 2,873,857, which issued to J. L. Schied on Feb. 17, 1959; 3,189,181, which issued to J. S. Couse on June 15, 1965; 3,247,971, which issued to R. F. Kastler on Apr. 16, 1966; 3,313,420, which issued to A. A. Hirsch on Apr. 11, 1967; 2,615,019, which issued to F. J. Early, Jr., on Oct. 26, 1972; 3,762,559, which issued to M. G. Knoy et al on Oct. 2, 1973; and 3,968,038, which issued to D. H. Nilsson on July 6, 1976.
In general, underdrain apparatuses or systems of the type disclosed by the prior art possess series flaws, including non-uniform or uneven backwash distribution which occurs because of the momentum of the water passing through a perforated header or channel. Water at a high velocity across an orifice will not be discharged through the orifice as readily as it will when flowing at a lower velocity across the orifice. Other problems include structural failures because the underdrain system is not sufficiently strong or securely anchored to resist the large upward thrust generated during a backwash operation.
Some underdrains are expensive to purchase and many are difficult and expensive to install and require tedious grouting procedures or cumbersome and expensive false bottom structures. Channelling and jetting and spouting bed action in the filter media occurs in many strainer type designs. Many underdrains have no ability to cope with trapped air which on explosive release is very disruptive in filters particularly those with gravel bedding. Many underdrains lack the flexibility to operate in the air scour assisted backwash mode or air scour simultaneous with backwash. Some types of underdrains, for example those of tile or porous tile, are quite fragile and much breakage during installation results. Inadequate corrosion resistance is a factor with some underdrains. Many types of underdrains require gravel layering as an inflexible requirement. Thus, in spite of the large number of different apparatuses or systems presently available, there is still much room for improvement in the filter underdrain field.
The object of the present invention is to meet the above need by providing a relatively simple filter underdrain apparatus, which substantially reduces the likelihood of most or any of the above mentioned problems being encountered.
While the term filter underdrain is used throughout for brevity, the application of the invention is not restricted by any means to filters only. There are various types of water/waste and process equipment that are not filters at all but where improved flow collection and backwash distribution would be most desirable. Examples of such equipment, and this list is by no means intended to be all encompassing, are:
upflow or downflow contact clarifiers or filters PA0 activated carbon contactors PA0 ion exchange units PA0 iron removal units - greensand/catalyzed sand/birm PA0 catalyst bed contactors, e.g. desilicizers PA0 neutralizing media contactors
Thus the term filter underdrains is used and understood to include units other than true filters.
In some process equipment vessels (upflow mode filters and contact clarifiers as examples) the underdrain serves a somewhat different function that in downflow, i.e. it serves to distribute incoming service flow as well as backwash.
Backwash in filters is clearly defined as a periodic reverse flow through the media to flow out trapped impurities. The term is used in ion exchange and carbon contactors as well, but means a somewhat different thing. In filters dirt is flushed from the bed by backwash. In ion exchange and carbon contactors, and the like, water is typically filtered in advance so backwash serves to "fluff up the bed" to eliminate packing and flow channelling so that contact is improved and short circuiting averted in carbon contactor units. In ion exchanger a "backwash" is required to wash any dirt from the bed, but more to "fluff the bed" so that regenerant contact is maximized, i.e. regenerant short circuiting avoided.